Satellites typically carry solar arrays that provide the power that the satellite requires to run its onboard systems. During eclipse periods however, the satellite draws energy from its batteries in order to provide power for the onboard systems. After the eclipse periods, when the solar array is once again exposed to light, the batteries are recharged using energy provided by the solar array. Thus, in the past, satellites have had to carry both solar arrays and batteries in order to provide energy to onboard systems.
The solar array and batteries also provide power to a mechanism which provides steering and pointing for the satellite. In the past, reaction wheels critically aligned with respect to three perpendicular axes have provided the steering and pointing functions. Rotating reaction wheels provide torques which depend in part on the speed and direction of rotation of the reaction wheel Known control laws are employed to adjust the reaction wheel speeds under the direction of, for example, a microprocessor, which is supplied power by the solar array and the batteries.
Batteries also require precise control over their charging profile in order to extend the battery life and thereby meet the satellite mission requirements. However, power generated by a solar array varies greatly from the beginning of life (BOL) of the solar array to the end of life (EOL) of the solar array. Because the batteries are charged with a fixed charge profile, power control circuitry must be included on each satellite to ensure that the power generated by the solar array is compatible with the charging voltage. For example, shunt regulators or series regulators are typically required to regulate power flow from the solar array and to provide the proper charging profile. In regulating the power flow from the solar array, shunt regulators simply dissipate excess current, typically through a resistor to ground. Therefore, although regulation is achieved, otherwise useful power is wasted. Series regulators, on the other hand, reduce available power to spacecraft loads due to internal losses in their circuitry.
Furthermore, reaction wheels, batteries, and the battery charge control circuitry take up a significant portion of the allocated satellite weight. Satellite batteries, in particular, are extremely heavy. As a result, a satellite cannot carry as much scientific, communications, or other equipment as it otherwise could. Furthermore, increased satellite weight also increases launch costs.
Therefore, a need remains for improved solar array regulation and satellite pointing which overcomes the disadvantages discussed above and previously experienced.